Three grades of milk are 1 percent, 2 percent, and 3 percent by volume. If x gallons of 1 percent grade, y gallons of 2 percent grade, z gallons of 3 percent grade are mixed to give x+y+z gallons of a 1.5 percent grade, what is x in terms of y and z?
A. y - 3 z
B. y + 3 z
C. 3 y - z
D. 3 y + z
E. 2 y + 3 z

OA B